The didemnins (FIG. 1) are a group of cyclic depsipeptides with extraordinary biological activities, including antitumor, antivirus and immunosuppressive activities (Vera and Joullié, 2002; Rawat et al., 2006). Rinehart and his coworkers (1981) initially isolated didemnins A, B and C from a Caribbean tunicate of the family Didemnidae in 1981. Since then, more than 20 other didemnins have been isolated from tunicates collected from different geographical locations. Owing to its potent antitumor activity, didemnin B was the first marine natural product selected to enter clinical trials. Although didemnin B failed in mid-1990s, a closely related didemnin compound-dehydrodidemnin B (Aplidin) is currently in human phase II clinic trials for solid and haematological malignant neoplasias like T cell lymphoma and myelofibrosis and in phase III clinical trials for multiple myeloma (Soto-Matos et al., 2011). Until now, didemnins are exclusively isolated from the eukaryotic marine tunicates. However, since cyclic peptides are usually synthesized by non-ribosomal peptide synthetases (NRPS) from microorganisms, a microorganism associated with the tunicates would be a useful source for producing didemnins. Until the present invention, careful scrutiny of tunicate symbiotic microorganisms has not been successful to locate such a microorganism and chemical synthesis remains to be the only route to obtain sufficient amount of didemnins. The chemical reactions to synthesize didemnin A, the simplest compound of the didemnin family, are too complicated to be finished within 10 steps (Jou et al, 1997), and consequently the yield is undoubtedly very low. Therefore, the discovery of didemnin biosynthetic gene cluster from a microbe will be beneficial for producing didemnins more economically. More importantly, novel didemnin analogues can be generated through genetic engineering of the biosynthetic pathways and be investigated for their biological activities, which may in turn provide more drug leads.